I am puzzled why a 12 volt battery terminal can get SO dirty, that it can cause a fault on an almost brand new car tbh ?.
Loose connection, Yes - Dirty connection - No surely ?.
It's very likely they just went ahead and did the 12 volt reset and returned the car.
Disconnected the negative terminal on the 12 volt battery, leave disconnected for about 20 minutes, then reconnect and tighten - job done.
Probably spent all day at the garage for 20 mins of work as well !.
How frustrating.
If you have a 10mm spanner, then try it yourself and save another lost day at the dealers.
Contact the service department and tell them the problem returned and you have done a 12 volt reset yourself to save time.
Ask them to log the problem on the system, just incase the problem returns again, then it will need further investigation.